To date, most implanted defibrillation electrode systems used in patients have consisted of two flexible metal braid electrodes with an insulated backing. These so called patches are placed at implant on the epicardial or pericardial surface and generally held in placed with sutures. A less frequently used alternative system of electrodes uses one patch electrode placed epicardially or pericardially, and a wound metallic ribbon electrode carried on a catheter placed in the superior vena cava (SVA). The wound electrode is commonly known as a "spring electrode".
Both the "patch-patch" electrode system and the "spring-patch" electrode system suffer from the drawback of requiring major surgery under general anesthesia to implant. Electrode systems which do not require major surgery to implant are presently being explored by a number of researchers.
One such system consists of a multi-electrode catheter and an optional external patch electrode which is placed in a subcutaneous space. The catheter carries three electrodes: (1) a distal small area button tip electrode, (2) a large area spring electrode located just proximal of the distal button, and (3) a large area spring electrode located at some distance proximal to the tip. The spacing between electrode (2) and electrode (3) is such that when electrodes (1) and (2) are in the right ventricle, and electrode (1) is at the right ventricular apex, then electrode (3) will be in the high right atrium and/or the SVC.
The electrical connections of the catheter for an automatic implantable cardioverter defibrillator (AICD) generally utilize electrodes (1) and (2) for pacing, fibrillation sensing and rate sensing, and electrodes (2) and (3) for cardioverting, and defibrillation.
The catheter of this system is known to have the following disadvantages:
1. The catheter is "stiff" at the electrodes. This lack of flexibility is primarily due to the properties of the metallic ribbon.
2. The catheter does not always retain its position in the heart once implanted, as there is no action fixation mechanism at the distal end. While passive fixation may be used, this does not insure acute stability, which is important if the lead is also used for pacing.
3. Since the spacing of the electrodes is fixed, the location of electrode (3) in of the heart varies from patient to patient due to variations in the size of the heart. This may lessen defibrillation efficacy in some patients.
4. Insulator break may occur when there are two separate and distinct shocking electrodes in the same lead body and high energy shocks of opposite polarity are carried.
5. Clotting around the large diameter electrode has been reported.